1. Field of the Invention
The present invention relates to a semiconductor light emitting device.
2. Description of the Related Art
Semiconductor light emitting devices can be small and highly effective in power consumption, and can emit vivid color light. In addition, in the case of light emitting devices of semiconductor devices, there are no concerns about bulb burnout and the like. In addition, semiconductor light emitting devices have features such as excellent initial driven characteristics, resistance to vibration or ON/OFF repeats. Since semiconductor light emitting devices have these excellent features, semiconductor light emitting devices such as light emitting diode (hereinafter, occasionally referred to as “LED”) and laser diode (hereinafter, occasionally referred to as “LD”) have been used as various types of light sources. In particular, in recent years, light emitting diodes receive attention as light source for lighting replacement of fluorescent lamps, in other words, as next-generation lighting having long life and low power consumption. From this viewpoint, further improvement is required in light output and light emission efficiency.
In order to improve outgoing efficiency of light emission, the inventors have been developed a flip-chip type light emitting device which has a reflective film with improved reflectivity, the flip-chip type light emitting device having electrode surfaces opposed to a mount surface (Laid-Open Patent Publication No. JP 2009-164,423). As shown in a cross-sectional view of FIG. 8A and an enlarged view of FIG. 8B, the light emitting device disclosed in this document includes a semiconductor structure 11 which has light-emitting layer 8, a light-outgoing surface 18 which is arranged on one of the main surfaces of the semiconductor structure 11, and electrodes 3 which are arranged on the other main surface opposed to the light-outgoing surface 18 and are electrically connected to the semiconductor structure 11. In the light emitting device, a reflecting structure 20 is formed between the semiconductor structure 11 and each of the electrodes 3. The reflecting structure 20 includes a reflective layer 16 which is formed above the semiconductor structure 11, and a dielectric multilayer film 4 which is arranged on this reflective layer 16 and is composed of a plurality of dielectric layers. The refractive index of the reflective layer 16 is smaller than the refractive index of the semiconductor structure 11. The center wavelength of the reflection spectrum of the reflecting structure 20 is longer than the light emission peak wavelength of the light-emitting layer 8. According this construction, it is possible to provide a reflecting structure which can be thin but can have excellent weatherability and high reflectivity.
See Japanese Laid-Open Patent Publication No. JP 2009-537,982 T.
As stated above, in the case of semiconductor light emitting devices, since there are no concerns about bulb burnout and the like as compared with light bulbs, in addition to this feature, further improvement is required in durability or reliability so that the semiconductor light emitting devices can be virtually maintenance-free. However, semiconductor light emitting devices have a disadvantage that the forward voltage in the operation will gradually rise as the use time of the semiconductor light emitting devices. If the forward voltage rises, the loss will be large so that the heat amount will be large. Heat dissipation is important for semiconductor light emitting devices. If the heat amount of semiconductor light emitting devices becomes large, this will affect the life of the products. Generally, if the forward voltage rises 10% or more, it is judged that failure occurs. In particular, in recent years, it is required to reduce power consumption. Also, from this viewpoint, increase of driving voltage is undesirable.
The present invention is aimed at solving the problem. It is a main object of the present invention to provide a semiconductor light emitting device which is excellent in life characteristic, and can suppress rise of forward voltage.